Semiconductor structure and manufacturing method thereof

ABSTRACT

The present disclosure provides a semiconductor device and a method for manufacturing the same. The semiconductor device comprises a substrate, a first III-V compound layer over the substrate, a second III-V compound layer on the first III-V compound layer, a third III-V compound layer on the second III-V compound layer, a source region on the third III-V compound layer, and a drain region on the third III-V compound layer. A percentage of aluminum of the third III-V compound layer is greater than that of the second III-V compound layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 15/017,234, filedon Feb. 5, 2016. All of the above-referenced applications are herebyincorporated herein by reference in their entirety.

FIELD

The present disclosure relates generally to a semiconductor structureand a method of manufacturing a semiconductor structure.

BACKGROUND

In semiconductor technology, due to their characteristics, GroupIII-Group V (or III-V) semiconductor compounds are used to form variousintegrated circuit devices, such as high power field-effect transistors,high frequency transistors, or high electron mobility transistors(HEMTs). A HEMT is a field effect transistor incorporating a junctionbetween two materials with different band gaps (i.e., a heterojunction)as the channel instead of a doped region, as is generally the case formetal oxide semiconductor field effect transistors (MOSFETs). Incontrast with MOSFETs, HEMTs have a number of attractive propertiesincluding high electron mobility, the ability to transmit signals athigh frequencies, etc.

From an application point of view, enhancement-mode (E-mode) HEMTs havemany advantages. E-mode HEMTs allow elimination of negative-polarityvoltage supply, and, therefore, reduction of the circuit complexity andcost. Despite the attractive properties noted above, a number ofchallenges exist in connection with developing III-V semiconductorcompound-based devices. Various techniques directed at configurationsand materials of these III-V semiconductor compounds have beenimplemented to try and further improve transistor device performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1A is a cross-sectional view of a semiconductor structure inaccordance with some embodiments of the present disclosure.

FIG. 1B is a graph showing a measurement result of the semiconductorstructure of FIG. 1A in accordance with some embodiments of the presentdisclosure.

FIG. 2 is a cross-sectional view of a semiconductor structure inaccordance with some embodiments of the present disclosure.

FIGS. 3A-3D are a series of cross-sectional views illustratingprocessing steps to fabricate the semiconductor structure, in accordancewith some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

FIG. 1A is a cross-sectional view of a semiconductor structure 1according to one or more embodiments of the present disclosure. In someembodiments, the semiconductor structure 1 may be a high electronmobility transistor (HEMT). The semiconductor structure 1 includes asubstrate 10, a buffer layer 11, a first III-V compound layer 12, asecond III-V compound layer 13, a first dielectric layer 14, a seconddielectric layer 18, a gate region 15, a source region 16, a drainregion 17 and isolation regions 19 a, 19 b.

In some embodiments, the substrate 10 includes a silicon carbide (SiC)substrate, sapphire substrate or a silicon substrate. The semiconductorstructure 1 also includes a heterojunction formed between two differentsemiconductor material layers, such as material layers with differentband gaps. For example, the semiconductor structure 1 includes anon-doped narrow-band gap channel layer and a wide-band gap n-typedonor-supply layer.

The buffer layer 11 is on the substrate 10. The buffer 11 acts as abuffer and/or a transition layer for the subsequently formed overlyinglayers. The buffer layer 11 may be epitaxially grown using Metal OrganicVapor Phase Epitaxy (MOVPE). The buffer layer 11 may function as aninterface to reduce lattice mismatch between substrate 10 and the firstIII-V compound layer 12. In some embodiments, the buffer layer 11includes an aluminum nitride (AlN) layer having a thickness in a rangebetween about 10 nanometers (nm) and about 300 nm. The buffer layer 11may include a single layer or a plurality of layers. In the case ofmultiple layers, the buffer layer 11 may include a low-temperature ALNlayer (not shown in the drawing) formed at a temperature between about800 Celsius degrees (° C.) and about 1,200° C., and a high-temperatureALN layer (not shown in the drawing) formed at a temperature betweenabout 1,000° C. and about 1,400° C.

The first III-V compound layer 12 is on the buffer layer 11. The firstIII-V compound layer 12 is a compound made from the III-V groups in theperiodic table of elements. In some embodiments of the presentdisclosure, the first III-V compound layer 12 includes a gallium nitride(GaN) layer. In some embodiments, the first III-V compound layer 12includes a GaAs layer or InP layer. In some embodiments, the first III-Vcompound layer 12 may be epitaxially grown by using, for example, MOVPE,during which a gallium-containing precursor and a nitrogen-containingprecursor are used. The gallium-containing precursor may includetrimethylgallium (TMG), triethylgallium (TEG), or other suitablegallium-containing chemicals. The nitrogen-containing precursor mayinclude ammonia (NH₃), tertiarybutylamine (TBAm), phenyl hydrazine, orother suitable chemicals.

The first III-V compound layer 12 is undoped. Alternatively, the firstIII-V compound layer 12 is unintentionally doped and may be, forexample, lightly doped with n-type dopants due to a precursor used toform the first III-V compound layer 12. In some embodiments, the firstIII-V compound layer 12 has a thickness in a range from about 0.5microns (μm) to about 10 μm.

The second III-V compound layer 13 is on the first III-V compound layer12. The second III-V compound layer 13 is a compound made from the III-Vgroups in the periodic table of elements. The second III-V compoundlayer 13 and the first III-V compound layer 12 are different from eachother in composition. In some embodiments of the present disclosure, thesecond III-V compound layer 13 includes an aluminum gallium nitride(AlGaN) layer. In some embodiments, the second III-V compound layer 13includes an AlGaAs layer or an AlInP layer. The second III-V compoundlayer 13 is intentionally doped. In some embodiments, the second III-Vcompound layer 13 has a thickness in a range from about 5 nm to about 50nm.

As shown in FIG. 1A, the second III-V compound layer 13 has two III-Vcompound layers 13 a and 13 b. In some embodiments, both of the III-Vcompound layers 13 a and 13 b are AlGaN with different percentages ofAl. Alternatively, both of the III-V compound layers 13 a and 13 b maybe AlGaAs or AlInP with different percentages of Al. The percentage ofAl of the III-V compound layer 13 a is lower than that of the III-Vcompound layer 13 b. In some embodiments, a ratio of the percentage ofAl of AlGaN of the III-V compound layer 13 b to that of the III-Vcompound layer 13 a is in a range from about 1.1 to 2.5. For example,the percentage of Al of AlGaN of the III-V compound layer 13 a is 16%and the percentage of Al of AlGaN of the III-V compound layer 13 b wouldbe in a range from about 17.6% to 40%. In some embodiments, thepercentage of Al of AlGaN of the III-V compound layer 13 a is in a rangefrom about 12% to about 18% while the percentage of Al of AlGaN of theIII-V compound layer 13 b is in a range from about 23% to about 40%.

In some embodiments, the second III-V compound layer 13 may have morethan two AlGaN layers, each having a different percentage of Al. In someembodiments, the percentage of Al of AlGaN layers increases as the AlGaNlayer is distant from the first III-V compound layer 12. Therefore, anAlGaN layer with a higher percentage of Al would be grown on an AlGaNlayer with a lower percentage of Al.

The isolation regions 19 a, 19 b are at both sides within the firstIII-V compound layer 12 and the second III-V compound layer 13. Theisolation regions 19 a, 19 b isolate the HEMT in the semiconductorstructure 1 from other devices in the substrate 10. In some embodiments,the isolation regions 19 a, 19 b include doped regions with species ofoxygen or nitrogen.

The source region 16 is on the III-V compound layer 13 b. In someembodiments, the source region 16 includes aluminum (Al), titanium (Ti),nickel (Ni), gold (Au) or copper (Cu). The drain region 17 is on theIII-V compound layer 13 b and spaced apart from the source region 16. Insome embodiments, the drain region 17 includes Al, Ti, Ni, Au or Cu.

The first dielectric layer 14 penetrates the III-V compound layer 13 band is located on the III-V compound layer 13 a. The first dielectriclayer 14 is configured to protect the underlying second III-V compoundlayer 13 from damage in the process having plasma. In some embodiments,the first dielectric layer 14 has a thickness in a range between about100 angstroms (Å) and about 5,000 Å. In some embodiments, the firstdielectric layer 14 may be a passivation layer including silicon oxideand/or silicon nitride. When comprising silicon nitride, the firstdielectric layer 14 may be formed by performing a Low-Pressure ChemicalVapor Deposition (LPCVD) method (without plasma) using SiH₄ and NH₃gases.

The gate region 15 is on the first dielectric layer 14 and between thesource region 16 and the drain region 17. The gate region 15 includes aconductive material layer configured for voltage bias. In someembodiments, the conductive material layer includes a refractory metalor its compounds, e.g., titanium (Ti), titanium nitride (TiN), titaniumtungsten (TiW) and tungsten (W). Alternatively, the conductive materiallayer includes nickel (Ni), gold (Au) or copper (Cu).

The second dielectric layer 18 is on the III-V compound layer 13 b andthe isolation layers 19 a, 19 b. The second dielectric layer 18surrounds the source region 16, the drain region 17, the firstdielectric layer 14 and the gate region 15. The second dielectric layer18 is configured to protect the underlying III-V compound layer 13 bfrom damage in the process having plasma. In some examples, the seconddielectric layer 18 may be a passivation layer including silicon oxide,silicon nitride, gallium oxide, aluminum oxide, scandium oxide,zirconium oxide, lanthanum oxide or hafnium oxide.

In some embodiments, the semiconductor structure 1 may further include aprotection layer (not shown in the drawing). The protection layer isdisposed between the source region 16 and the second dielectric layer 18and between the drain region 17 and the second dielectric layer 18. Theprotection layer covers the source region 16 and the drain region 17 toprevent the source region 16 and the drain region 17 from exposureduring an annealing process in the formation of the isolation regions 19a, 19 b.

In some existing approaches, a III-V compound semiconductor structureonly has a single AlGaN layer. Therefore, the percentage of Al of AlGaNlayer is a critical parameter to determine the performance thesemiconductor structure. In the case of low percentage of Al, electronswould be trapped easily during the operation of the semiconductorstructure due to the low energy barrier, which would increaseon-resistance (Ron) of the semiconductor structure and decay theefficiency of the semiconductor structure. Although increasing thepercentage of Al of the AlGaN layer may solve the above issue, thehigher percentage of Al of the AlGaN layer would result in a highlattice mismatch between the AlGaN layer and the underlying GaN layer,which would reduce the reliability of the semiconductor structure.Therefore, it is difficult to obtain a III-V compound semiconductorstructure with low on-resistance and high reliability by using a singleAlGaN layer.

In accordance with some embodiments of the present disclosure, thesemiconductor structure has multiple AlGaN layers, each having adifferent percentage of Al. By growing a AlGaN layer 13 a with a lowerpercentage of Al directly on the GaN layer, the lattice mismatch betweenthe AlGaN layer and the GaN layer would be reduced. Growing anotherAlGaN layer 13 b with a higher percentage of Al on the AlGaN layer 13 awould increase the energy barrier to avoid the electrons from beingtrapped easily, which would reduce the n-resistance of the semiconductorstructure 1. Therefore, in comparison with the existing semiconductorstructure with a single AlGaN layer, the semiconductor structure 1 haslower on-resistance and higher efficiency without reducing thereliability.

FIG. 1B illustrates a measurement result of the semiconductor structure1 in FIG. 1A, in accordance with some embodiments. In FIG. 1B, thex-axis represents a reverse bias stress voltage applied to thesemiconductor structure 1, and the y-axis represents a dynamicon-resistance ratio that is a ratio between the on-resistance of thesemiconductor structure to which a reverse bias stress voltage isapplied and the on-resistance of the semiconductor structure to which areverse bias stress voltage is not applied.

As shown in FIG. 1B, under 300 V stress voltage, the dynamicon-resistance ratio of the semiconductor structure 1 is about 1.1, whichis about 25% lower than that of the existing semiconductor structurewith a single AlGaN layer. Under 400 V stress voltage, the dynamicon-resistance ratio of the semiconductor structure 1 is about 1.8, whichis about 62% lower than that of the existing semiconductor structurewith a single AlGaN layer. Lower dynamic on-resistance ratio wouldincrease the efficiency and the performance of the semiconductorstructure 1.

FIG. 2 is a cross-sectional view of a semiconductor structure 2according to one or more embodiments of the present disclosure. In someembodiments, the semiconductor structure 2 may be a high electronmobility transistor (HEMT). The semiconductor structure 2 includes asubstrate 20, a buffer layer 21, a first III-V compound layer 22, asecond III-V compound layer 23, a first dielectric layer 24, a seconddielectric layer 28, a gate region 25, a source region 26, a drainregion 27 and isolation regions 29 a, 29 b.

In some embodiments, the substrate 20 includes a silicon carbide (SiC)substrate, sapphire substrate or a silicon substrate. The semiconductorstructure 2 also includes a heterojunction formed between two differentsemiconductor material layers, such as material layers with differentband gaps. For example, the semiconductor structure 2 includes anon-doped narrow-band gap channel layer and a wide-band gap n-typedonor-supply layer.

The buffer layer 21 is on the substrate 20. The buffer layer 21 acts asa buffer and/or a transition layer for the subsequently formed overlyinglayers. The buffer layer 21 may be epitaxially grown using Metal OrganicVapor Phase Epitaxy (MOVPE). The buffer layer 21 may function as aninterface to reduce lattice mismatch between substrate 20 and the firstIII-V compound layer 22. In some embodiments, the buffer layer 21includes an aluminum nitride (ALN) layer having a thickness in a rangebetween about 10 nanometers (nm) and about 300 nm. The buffer layer 21may include a single layer or a plurality of layers. For example, thebuffer layer 21 may include a low-temperature ALN layer (not shown inthe drawing) formed at a temperature between about 800° C. and about1,200° C., and a high-temperature ALN layer (not shown in the drawing)formed at a temperature between about 1,000° C. and about 1,400° C.

The first III-V compound layer 22 is on the buffer layer 21. The firstIII-V compound layer 22 is a compound made from the III-V groups in theperiodic table of elements. In some embodiments of the presentdisclosure, the first III-V compound layer 22 includes a gallium nitride(GaN) layer. In some embodiments, the first III-V compound layer 22includes a GaAs layer or InP layer. In some embodiments, the first III-Vcompound layer 22 may be epitaxially grown by using, for example, MOVPE,during which a gallium-containing precursor and a nitrogen-containingprecursor are used. The gallium-containing precursor may includetrimethylgallium (TMG), triethylgallium (TEG), or other suitablegallium-containing chemicals. The nitrogen-containing precursor mayinclude ammonia (NH₃), tertiarybutylamine (TBAm), phenyl hydrazine, orother suitable chemicals.

The first III-V compound layer 22 is undoped. Alternatively, the firstIII-V compound layer 22 is unintentionally doped and may be, forexample, lightly doped with n-type dopants due to a precursor used toform the first III-V compound layer 22. In some embodiments, the firstIII-V compound layer 22 has a thickness in a range from about 0.5microns (μm) to about 10 μm.

The second III-V compound layer 23 is on the first III-V compound layer22 to cover a portion of the first III-V compound layer 22. The secondIII-V compound layer 23 is a compound made from the III-V groups in theperiodic table of elements. The second III-V compound layer 23 and thefirst III-V compound layer 22 are different from each other incomposition. In some embodiments of the present disclosure, the secondIII-V compound layer 23 includes an aluminum gallium nitride (AlGaN)layer. In some embodiments, the second III-V compound layer 23 includesan AlGaAs layer or an AlInP layer. The second III-V compound layer 23 isintentionally doped. In some embodiments, the second III-V compoundlayer 23 has a thickness in a range from about 5 nm to about 50 nm.

As shown in FIG. 2, the second III-V compound layer 23 has two III-Vcompound layers 23 a and 23 b. In some embodiments, both of the III-Vcompound layers 23 a and 23 b are AlGaN with different percentages ofAl. Alternatively, both of the III-V compound layers 23 a and 23 b maybe AlGaAs or AlInP with different percentages of Al. The percentage ofAl of the III-V compound layer 23 a is lower than that of the III-Vcompound layer 23 b. In some embodiments, a ratio of the percentage ofAl of AlGaN of the III-V compound layer 23 b to that of the III-Vcompound layer 23 a is in a range from about 1.1 to 2.5. For example,the percentage of Al of AlGaN of the III-V compound layer 23 a is 16%and the percentage of Al of AlGaN of the III-V compound layer 23 b wouldbe in a range from about 17.6% to 40%. In some embodiments, thepercentage of Al of AlGaN of the III-V compound layer 23 a is in a rangefrom about 12% to about 18% while the percentage of Al of AlGaN of theIII-V compound layer 23 b is in a range from about 23% to about 40%.

In some embodiments, the second III-V compound layer 23 may have morethan two AlGaN layers, each having a different percentage of Al. In someembodiments, the percentage of Al of AlGaN layers increases as the AlGaNlayer is distant from the first III-V compound layer 22. Therefore, anAlGaN layer with a higher percentage of Al would be grown on an AlGaNlayer with a lower percentage of Al.

The isolation regions 29 a, 29 b are at both sides within the firstIII-V compound layer 22 and the second III-V compound layer 23. Theisolation regions 29 a, 29 b isolate the HEMT in the semiconductorstructure 2 from other devices in the substrate 20. In some embodiments,the isolation regions 29 a, 29 b include doped regions with species ofoxygen or nitrogen.

The source region 26 is on the III-V compound layer 23 b. In someembodiments, the source region 26 includes aluminum (Al), titanium (Ti),nickel (Ni), gold (Au) or copper (Cu). The drain region 27 is on theIII-V compound layer 23 b and spaced apart from the source region 26. Insome embodiments, the drain region 27 includes Al, Ti, Ni, Au or Cu.

The first dielectric layer 24 is located on the III-V compound layer 23a and spaced apart from the III-V compound layer 23 b. A ratio of adistance L2 between the first dielectric layer 24 and the III-V compoundlayer 23 b beneath the drain region 27 to a distance L1 between thefirst dielectric layer 24 and the drain region 27 is in a range from 0to about 0.8. In some embodiments, L1 is about 15 μm and L2 is less than12 μm. The first dielectric layer 24 is configured to protect theunderlying second III-V compound layer 23 from damage in the processhaving plasma. In some embodiments, the first dielectric layer 24 has athickness in a range between about 100 Å and about 5,000 Å. In someembodiments, the first dielectric layer 24 may be a passivation layerincluding silicon oxide and/or silicon nitride. When comprising siliconnitride, the first dielectric layer 24 may be formed by performing aLow-Pressure Chemical Vapor Deposition (LPCVD) method (without plasma)using SiH₄ and NH₃ gases.

The gate region 25 is on the first dielectric layer 24 and between thesource region 26 and the drain region 27. The gate region 25 includes aconductive material layer configured for voltage bias. In someembodiments, the conductive material layer includes a refractory metalor its compounds, e.g., titanium (Ti), titanium nitride (TiN), titaniumtungsten (TiW) and tungsten (W). Alternatively, the conductive materiallayer includes nickel (Ni), gold (Au) or copper (Cu).

The second dielectric layer 28 is on the III-V compound layer 23 b, theIII-V compound layer 23 a and the isolation layers 19 a, 19 b. Thesecond dielectric layer 28 surrounds the source region 26, the drainregion 27, the first dielectric layer 24 and the gate region 25. Thesecond dielectric layer 28 is configured to protect the underlying III-Vcompound layer 23 b from damage in the process having plasma. In someexamples, the second dielectric layer 28 may be a passivation layerincluding silicon oxide, silicon nitride, gallium oxide, aluminum oxide,scandium oxide, zirconium oxide, lanthanum oxide or hafnium oxide.

In some embodiments, the semiconductor structure 2 may further include aprotection layer (not shown in the drawing). The protection layer isdisposed between the source region 26 and the second dielectric layer 28and between the drain region 27 and the second dielectric layer 28. Theprotection layer covers the source region 26 and the drain region 27 toprevent the source region 26 and the drain region 27 from exposureduring an annealing process in the formation of the isolation regions 29a, 29 b.

In some existing approaches, a III-V compound semiconductor structureonly has a single AlGaN layer. Therefore, the percentage of Al of AlGaNlayer is a critical parameter to determine the performance thesemiconductor structure. In the case of low percentage of Al, electronswould be trapped easily during the operation of the semiconductorstructure due to the low energy barrier, which would increaseon-resistance (Ron) of the semiconductor structure and decay theefficiency of the semiconductor structure. Although increasing thepercentage of Al of the AlGaN layer may solve the above issue, thehigher percentage of Al of the AlGaN layer would result in a highlattice mismatch between the AlGaN layer and the underlying GaN layer,which would reduce the reliability of the semiconductor structure.Therefore, it is difficult to obtain a III-V compound semiconductorstructure with low on-resistance and high reliability by using a singleAlGaN layer.

In accordance with some embodiments of the present disclosure, thesemiconductor structure has multiple AlGaN layers, each having adifferent percentage of Al. By growing a AlGaN layer 23 a with a lowerpercentage of Al directly on the GaN layer, the lattice mismatch betweenthe AlGaN layer and the GaN layer would be reduced. Growing anotherAlGaN layer 23 b with a higher percentage of Al on the AlGaN layer 23 awould increase the energy barrier to avoid the electrons from beingtrapped easily, which would reduce the on-resistance of thesemiconductor structure 2. Therefore, in comparison with the existingsemiconductor structure with a single AlGaN layer, the semiconductorstructure 2 has lower on-resistance and higher efficiency withoutreducing the reliability. In addition, in comparison with thesemiconductor structure 1 shown in FIG. 1A, the semiconductor structure2 may have better performance by separating the III-V compound layer 23b from the first dielectric layer 24.

FIG. 3A to FIG. 3D are cross-sectional views of a semiconductorstructure 3 fabricated at various stages, in accordance with someembodiments of the present disclosure. Various figures have beensimplified for a better understanding of the inventive concepts of thepresent disclosure.

Referring to FIG. 3A, a substrate 30 is provided. The substrate 30includes a silicon carbide (SiC) substrate, sapphire substrate or asilicon substrate. The semiconductor structure 3 also includes aheterojunction formed between two different semiconductor materiallayers, such as material layers with different band gaps. For example,the semiconductor structure 3 includes a non-doped narrow-band gapchannel layer and a wide-band gap n-type donor-supply layer.

The buffer layer 31 is formed on the substrate 30. The buffer layer 31acts as a buffer and/or a transition layer for the subsequently formedoverlying layers. The buffer layer 31 may be epitaxially grown usingMetal Organic Vapor Phase Epitaxy (MOVPE). The buffer layer 31 mayfunction as an interface to reduce lattice mismatch between substrate 30and the subsequently formed III-V compound layer. In some embodiments,the buffer layer 31 includes an aluminum nitride (ALN) layer having athickness in a range between about 10 nanometers (nm) and about 300 nm.The buffer layer 31 may include a single layer or a plurality of layers.For example, the buffer layer 31 may include a low-temperature ALN layer(not shown in the drawing) formed at a temperature between about 800° C.and about 1,200° C., and a high-temperature ALN layer (not shown in thedrawing) formed at a temperature between about 1,000° C. and about1,400° C.

The first III-V compound layer 32 is formed on the buffer layer 31. Thefirst III-V compound layer 32 is a compound made from the III-V groupsin the periodic table of elements. In some embodiments of the presentdisclosure, the first III-V compound layer 32 includes a gallium nitride(GaN) layer. In some embodiments, the first III-V compound layer 32includes a GaAs layer or InP layer. In some embodiments, the first III-Vcompound layer 32 may be epitaxially grown by using, for example, MOVPE,during which a gallium-containing precursor and a nitrogen-containingprecursor are used. The gallium-containing precursor may includetrimethylgallium (TMG), triethylgallium (TEG), or other suitablegallium-containing chemicals. The nitrogen-containing precursor mayinclude ammonia (NH₃), tertiarybutylamine (TBAm), phenyl hydrazine, orother suitable chemicals.

The first III-V compound layer 32 is undoped. Alternatively, the firstIII-V compound layer 32 is unintentionally doped and may be, forexample, lightly doped with n-type dopants due to a precursor used toform the first III-V compound layer 32. In some embodiments, the firstIII-V compound layer 32 has a thickness in a range from about 0.5 μm toabout 10 μm.

The second III-V compound layer 33 a is formed on the first III-Vcompound layer 32. The second III-V compound layer 33 a is a compoundmade from the III-V groups in the periodic table of elements. The secondIII-V compound layer 33 a and the first III-V compound layer 32 aredifferent from each other in composition. In some embodiments of thepresent example, the second III-V compound layer 33 a includes analuminum gallium nitride (AlGaN) layer. In some embodiments, the secondIII-V compound layer 33 a includes an AlGaAs layer or an AlInP layer.The second III-V compound layer 33 a is intentionally doped. The secondIII-V compound layer 33 a is epitaxially grown on the first III-Vcompound layer 32 by MOVPE using aluminum-containing precursor,gallium-containing precursor, and nitrogen-containing precursor. Thealuminum-containing precursor includes trimethylaluminum (TMA),triethylaluminium (TEA), or other suitable chemical. Thegallium-containing precursor includes TMG, TEG, or other suitablechemical. The nitrogen-containing precursor includes ammonia, TBAm,phenyl hydrazine, or other suitable chemical.

The third III-V compound layer 33 b is formed on the second III-Vcompound layer 33 a. The third III-V compound layer 33 b and the secondIII-V compound layer 33 a are formed of same compound except that thepercentage of Al of the third compound layer 33 b is different from thatof the second compound layer 33 a. More specifically, the percentage ofAl of the third compound layer 33 b is higher than that of the secondcompound layer 33 a. In some embodiments, a ratio of the percentage ofAl of AlGaN of the third III-V compound layer 33 b to that of the secondIII-V compound layer 33 a is in a range from about 1.1 to 2.5. Forexample, the percentage of Al of AlGaN of the second III-V compoundlayer 33 a is 16% and the percentage of Al of AlGaN of the third III-Vcompound layer 33 b would be in a range from about 17.6% to 40%. In someembodiments, the percentage of Al of AlGaN of the III-V compound layer33 a is in a range from about 12% to about 18% while the percentage ofAl of AlGaN of the III-V compound layer 33 b is in a range from about23% to about 40%.

The third III-V compound layer 33 b is epitaxially grown on the secondIII-V compound layer 33 a by MOVPE using aluminum-containing precursor,gallium-containing precursor, and nitrogen-containing precursor. Thealuminum-containing precursor includes trimethylaluminum (TMA),triethylaluminium (TEA), or other suitable chemical. Thegallium-containing precursor includes TMG, TEG, or other suitablechemical. The nitrogen-containing precursor includes ammonia, TBAm,phenyl hydrazine, or other suitable chemical.

The isolation regions 39 a, 39 b are formed at both sides within thefirst III-V compound layer 32, the second III-V compound layer 33 a andthe third III-V compound layer 33 b. In some embodiments, the isolationregions 39 a, 39 b are formed by an implantation process with species ofoxygen or nitrogen.

Referring to FIG. 3B, a source region 36 and a drain region 37 areformed on the third III-V compound layer 33 b. In some embodiments, thesource region 36 and the drain region 37 are formed by using sputtering,atomic layer deposition (ALD) or physical vapor deposition (PVD)operations. In some embodiments, the source region 36 and the drainregion 37 include Au, Al, Ti, Ni, Au or Cu.

Referring to FIG. 3C, an opening is formed from the top surface of thethird III-V compound layer 33 b to the second III-V compound layer 33 a.The opening is defined by lithography and etching processes to exposethe top surface of the second III-V compound layer 33 a. The firstdielectric layer 34 is formed within the opening and on the top surfaceof the second III-V compound layer 33 a. In some embodiments, the firstdielectric layer 34 includes silicon oxide and/or silicon nitride. Insome embodiments, the first dielectric layer 34 is formed by performinga low pressure chemical vapor deposition (LPCVD) method without plasmausing SiH₄ and NH₃ gases. An operation temperature is in a range of fromabout 650° C. to about 800° C. An operation pressure is in a range ofabout 0.1 Torr and about 1 Torr.

Then a metal layer is deposited in the first dielectric layer 34 to formthe gate region 35. In some embodiments, the gate region 35 includes arefractory metal or its compounds, e.g., titanium (Ti), titanium nitride(TiN), titanium tungsten (TiW) and tungsten (W). Alternatively, the gateregion 35 includes nickel (Ni), gold (Au) or copper (Cu).

Referring to FIG. 3D, a second dielectric layer 38 is deposited on thesource region 36, the drain region 37, the gate region 35 and the thirdIII-V compound layer 33 b. In some examples, the second dielectric layer38 comprises silicon oxide, silicon nitride, gallium oxide, aluminumoxide, scandium oxide, zirconium oxide, lanthanum oxide or hafniumoxide. In some embodiments, the second dielectric layer 38 is formed byan atomic layer deposition (ALD) method. The ALD method is based on thesequential use of a gas phase chemical process. The majority of ALDreactions use two chemicals, typically called precursors. Theseprecursors react with a surface one-at-a-time in a sequential manner. Byexposing the precursors to the growth surface repeatedly, the seconddielectric layer 38 is deposited. The ALD method provides a uniformthickness of the second dielectric layer 38 with high quality. In someembodiments, the second dielectric layer 38 is formed by a plasmaenhanced chemical vapor deposition (PECVD) or a low pressure chemicalvapor deposition (LPCVD).

As stated above, the existing III-V compound semiconductor structurecannot achieve low on-resistance and high reliability simultaneously byusing a single AlGaN layer. In accordance with some embodiments of thepresent disclosure, by epitaxially growing multiple AlGaN layers withdifferent percentage of Al on the GaN layer, the semiconductor structuremanufactured by the operations shown in FIG. 3A to FIG. 3D would havelow on-resistance and high efficiency without reducing the reliability.

One embodiment of the present disclosure provides a semiconductordevice. The semiconductor device comprises a substrate, a first III-Vcompound layer over the substrate, a second III-V compound layer on thefirst III-V compound layer, a third III-V compound layer on the secondIII-V compound layer, a source region on the third III-V compound layer,and a drain region on the third III-V compound layer. A percentage ofaluminum of the third III-V compound layer is greater than that of thesecond III-V compound layer.

One embodiment of the present disclosure provides a high electronmobility transistor (HEMT). The HEMT comprises a substrate, a firstIII-V compound layer over the substrate, a second III-V compound layeron the first III-V compound layer, a third III-V compound layer on thesecond III-V compound layer, a source region on the third III-V compoundlayer, a drain region on the third III-V compound layer, a firstdielectric layer penetrating the third III-V compound layer and on thesecond III-V compound layer, and a gate region on the first dielectriclayer. A ratio of the distance between the first dielectric layer andthe third III-V compound layer beneath the drain region to a distancebetween the first dielectric layer and the drain region is in a rangefrom 0 to about 0.8.

One embodiment of the present disclosure provides a method ofmanufacturing a semiconductor device. The method comprises providing asubstrate, forming a first III-V compound layer over the substrate,forming a second III-V compound layer with a first concentration of Alon the first III-V compound layer, forming a third III-V compound layerwith a second concentration of Al on the second III-V compound layer,forming a source region on the third III-V compound layer, and forming adrain region on the third III-V compound layer. The second concentrationof Al is higher than the first concentration of Al.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, and composition of matter, means, methods and stepsdescribed in the specification. As those skilled in the art will readilyappreciate from the disclosure of the present disclosure, processes,machines, manufacture, composition of matter, means, methods or stepspresently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure. Accordingly, the appended claims are intended toinclude within their scope such as processes, machines, manufacture,compositions of matter, means, methods or steps/operations. In addition,each claim constitutes a separate embodiment, and the combination ofvarious claims and embodiments are within the scope of the disclosure.

What is claimed is:
 1. A method of manufacturing a semiconductor device,the method comprising: providing a substrate; epitaxially growing afirst III-V compound layer over the substrate; epitaxially growing asecond III-V compound layer with a first concentration of Al on thefirst III-V compound layer; epitaxially growing a third III-V compoundlayer with a second concentration of Al on the second III-V compoundlayer; forming a source region on the third III-V compound layer; andforming a drain region on the third III-V compound layer, wherein thesecond concentration of Al is higher than the first concentration of Al.2. The method of claim 1, wherein the second III-V compound layer andthe third compound layer are epitaxially grown using Metal Organic VaporPhase Epitaxy (MOVPE).
 3. The method of claim 1, wherein a ratio of thesecond concentration of Al to the first concentration of Al is in arange from about 1.1 to about 2.5.
 4. The method of claim 1, furthercomprising: forming an opening from a top surface of the third III-Vcompound layer to expose a top surface of the third III-V compoundlayer; filling the opening to form a dielectric layer; and forming agate region on the dielectric layer, wherein a ratio of the distancebetween the dielectric layer and the third III-V compound layer beneaththe drain region to a distance between the dielectric layer and thedrain region is in a range from 0 to about 0.8.
 5. A method ofmanufacturing a high electron mobility transistor (HEMT), comprising:providing a substrate; epitaxially growing a first III-V compound layerover the substrate; epitaxially growing a second III-V compound layer onthe first III-V compound layer; epitaxially growing a third III-Vcompound layer on the second III-V compound layer; forming a sourceregion on the third III-V compound layer; forming a drain region on thethird III-V compound layer; depositing a first dielectric layer on thesecond III-V compound layer through the third III-V compound layer; andforming a gate region on the first dielectric layer, wherein a side wallof the third III-V compound layer is separated from a side wall of thefirst dielectric layer by a first lateral distance; a side wall of theentirety of the drain region facing the gate region is separated fromthe side wall of the first dielectric layer by a second lateraldistance; the side wall of the third III-V compound layer, the side wallof the drain region and the side wall of the first dielectric layer areon a same side of the gate region; and the second lateral distance islarger than the first lateral distance.
 6. The method of claim 5,wherein the epitaxially growing the second and third III-V compoundlayers comprises: epitaxially growing the third III-V compound layerhaving a percentage of aluminum (Al) greater than that of the secondIII-V compound layer.
 7. The method of claim 5, wherein a ratio of apercentage of Al of the third III-V compound layer to that of the secondIII-V compound layer is in a range from about 1.1 to about 2.5.
 8. Themethod of claim 5, wherein the epitaxially growing the second III-Vcompound layer comprises: epitaxially growing the second III-V compoundlayer having a percentage of Al in a range from about 12% to about 18%.9. The method of claim 5, wherein the epitaxially growing the thirdIII-V compound layer comprises: epitaxially growing the third III-Vcompound layer having a percentage of Al in a range from about 23% toabout 40%.
 10. The method of claim 5, wherein the second lateraldistance is furthest between the drain region and the first dielectriclayer; and a ratio of the first lateral distance to the second lateraldistance is about 0.8.
 11. The method of claim 5, wherein theepitaxially growing the first III-V compound layer comprises:epitaxially growing the first III-V compound layer includes GaN, GaAs orInP; and the epitaxially growing the second and third III-V compoundlayers comprises: epitaxially growing the second and third III-Vcompound layers including AlGaN, AlGaAs or AlInP.
 12. The method ofclaim 5, wherein the third III-V compound layer is free from overlappingthe first dielectric layer from a top view perspective.
 13. A method ofmanufacturing a high electron mobility transistor (HEMT), comprising:providing a substrate; epitaxially growing a first III-V compound layerover the substrate; epitaxially growing a second III-V compound layer onthe first III-V compound layer; epitaxially growing a third III-Vcompound layer on the second III-V compound layer; forming a sourceregion on the third III-V compound layer; forming a drain region on thethird III-V compound layer; depositing a first dielectric layer arrangedon the second III-V compound layer through the third III-V compoundlayer; forming a gate region on the first dielectric layer, wherein thethird III-V compound layer is free from overlapping the first dielectriclayer from a top view perspective, the third III-V compound layer is indirect contact with the second III-V compound layer, and a percentage ofaluminum (Al) of the third III-V compound layer is greater than that ofthe second III-V compound layer; the third III-V compound layercomprises laterally separated first portion and second portions arrangedon a first side and a second side of the gate region, respectively, thesource region and the drain region reside on the first side and thesecond side of the gate region, respectively, a side wall of the firstportion of the third III-V compound layer and a first side wall of thefirst dielectric layer are laterally separated from each other by afirst distance, a side wall of the second portion of the third III-Vcompound layer and a second side wall of first dielectric layer arelaterally separated from each other by a second distance, the first sidewall and the second side wall of the first dielectric layer are arrangedon the first side and the second side of the gate region, respectively,the first distance is smaller than the second distance.
 14. The methodof claim 13, wherein a side wall of a first region of the drain regionand the source region is laterally separated from one of the first sidewall and the second side wall of the first dielectric layer by a thirddistance; the side wall of the first region and the one of the firstside wall and the second side wall of the first dielectric layer are ona same side of the gate region; and the third distance is larger thanone of the first distance and the second distance corresponding to theone of the first side wall and the second side wall of the firstdielectric layer.
 15. The method of claim 14, wherein the third distanceis furthest between the drain region and the first dielectric layer; anda ratio of the second distance to the third distance is about 0.8. 16.The method of claim 13, wherein a ratio of a percentage of Al of thethird III-V compound layer to that of the second III-V compound layer isin a range from about 1.1 to about 2.5.
 17. The method of claim 13,wherein the epitaxially growing the second III-V compound layercomprises: epitaxially growing the second III-V compound layer having apercentage of Al in a range from about 12% to about 18%.
 18. The methodof claim 13, wherein the epitaxially growing the third III-V compoundlayer comprises: epitaxially growing the third III-V compound layerhaving a percentage of Al in a range from about 23% to about 40%. 19.The method of claim 13, wherein the epitaxially growing the first III-Vcompound layer comprises: epitaxially growing the first III-V compoundlayer includes GaN, GaAs or InP.
 20. The method of claim 13, wherein theepitaxially growing the second and third III-V compound layerscomprises: epitaxially growing the second and third III-V compoundlayers including AlGaN, AlGaAs or AlInP.